This invention relates to a process of registration in a color xerographic printing system, and more particularly, to a self registration process in a Discharge Area Develop (DAD) system which utilizes a marking system for monitoring the toner placement of the first latent image for precise placement of toner on the subsequent latent images. It should be noted that hereinafter, for the purpose of simplicity, the "color xerographic printing system" is referred to as "color printing system".
Referring to FIG. 1, a color printing system 10 comprises a photoreceptor 12, and four color stations. However, for the purpose of simplicity, only two stations A.sub.1 and A.sub.2 are shown. Each color station, which is dedicated to a single color, comprises a charger 14, a raster output scanner (ROS) 16 and a developer 18. A charger is a device which charges the photoreceptor evenly prior to scanning, a ROS is a system which generates a latent image and a developer is a device which holds toner and deposits toner onto the latent image.
Referring to FIG. 2, there is shown a tangential (fast-scan) view of the raster output scanner 16 of the printing system 10 of FIG. 1. The raster scanning system 16 utilizes a laser light source 20, a collimator 22, pre-polygon optics 24, a multi-faceted rotating polygon mirror 26 as the scanning element, post polygon optics 28 and a photosensitive medium 30.
The laser light source 20 sends a light beam 32 to the rotating polygon mirror 26 through the collimator 22 and the pre-polygon optics 24. The collimator 22 collimates the light beam 32 and the pre-polygon optics 24 focuses the light beam 32 in the sagittal or cross-scan plane onto the rotating polygon mirror 28. The facets 34 of the rotating polygon mirror 26 reflect the light beam 32 and also cause the reflected light beam 32 to revolve about an axis near the reflection point of the facet 34. The reflected light beam 32 is utilized through the post polygon optics 28 to scan a photosensitive medium 30, such as a xerographic drum (photoreceptor). Referring to FIG. 3, since the photoreceptor 30 moves, the light beam 32 scans all the scan lines 36 of a document on the photoreceptor and generates a latent image.
Typically, in a color printing system, a latent image is being generated for each basic color and each latent image is being placed over the previous latent images. Referring to both FIGS. 1 and 3, each one of the color stations A.sub.1 and A.sub.2 generates one of the latent images. In the ROS 16 of each color station, the scanning light beam is modulated by the information of a given color. The scanning light beam 32 scans the photoreceptor 30 and discharges the photoreceptor according to the information of the given color to generate a latent image for the given color. While a latent image is being generated, the portion that is discharged will move under the developer 18 to be developed. Developing is defined as the latent image attracting toner from a toner station.
As the first latent image is being developed, the developed portion of the first latent image gradually moves into color station A.sub.2. In this ROS 16 of the color station A.sub.2, the light beam will be modulated by the information of a different color. The modulated light beam will start generating a new latent image over the first latent image.
In this manner each one of the following stations generates and develops a latent image over the previous latent images. The process of generating and developing a latent image is repeated four times, each by one of the stations, for four different colors (typically, cyan, yellow magenta and black). After the four different color toners are placed over each other, the toners will be transferred onto a sheet of paper.
Since each latent image is being generated over the previous latent image, the placement of each latent image is very critical. However, due to several factors such as the photoreceptor motion variation, vibration, thermal expansion, etc., the location of the scan lines of the latent images following the first latent image might be slightly different compared to the position of the scan lines of first latent image. Slight variation of the location of the scan lines causes the pixels of the same scan line of each latent image to be placed at different locations. This causes a problem known as mis-regiteration.
Referring to FIG. 4, one approach to match the scan lines of each latent image with the same scan lines of the first latent image is to place registration marks 40 and 42 on the margins 44 and 46 of each scan line, on the first latent image. Referring back to FIG. 1, the registration marks 40 and 42 will be developed as the first latent image is being developed. During the generation of the following latent images, the developed registration marks 40 and 42 of the developed first latent image (primary image 45) will be checked to determine if the current scan line is offset compared to the same scan line of the first latent image.
Checking or reading the registration marks can be achieved through different means such as CCD cameras, pattern recognition software and slit detectors.
When the scanning light beam is used to read the developed registration marks, the reflection of the light beam from the photoreceptor will be used. Referring to FIG. 5, the photoreceptor 30 reflects the light beam 32 (shown by solid line) where it does not have toner and absorbs the light beam 32' (shown by dashed line) where it has toner 43. This concept is used to read the registration marks. Two sensors 50 and 52 are placed over the photoreceptor on each margin 44 and 46 respectively to detect where the light beam is not reflected back (such as developed registration mark 43) to identify the developed registration marks on the margins.
During the generation of the following latent images, the scanning light beam will read the developed registration marks to compare the position of the current scan line with the position of the same scan line of the primary image 45. This method is appropriate for Charged Area Develop (CAD) which is also referred to as write-white as disclosed in U.S. Pat. No. 5,255,154. However, in the Discharged Area Develop (DAD) printing system which is also referred to as write-black, this method is not appropriate.
In a raster scanning system using DAD printing system, when a light beam strikes the photoreceptor, it discharges the photoreceptor. As a result, once a light beam scans over the developed registration marks for the purpose of detecting them, it will again discharge the photoreceptor under the developed registration marks. Therefore, the developed registration marks will attract more toner during the time the current latent image is being developed. The attraction of more toners on the registration marks is undesirable since more toner causes the marks to become large and loose their accuracy.
It is an object of this invention to provide a method of generating and reading the developed registration marks by a scanning light beam in a DAD printing system without attracting extra toner on the developed registration marks during the generation of the following latent images.